In the operation of metallurgical furnaces, e.g., electric arc furnaces, the graphite electrode is generally used because its mechanical and electrical properties are suited to tolerate the adverse conditions within this type of furnace. Nevertheless, the electrodes should moreover tolerate consumption due to erosion, corrosion, oxidation, sublimation by arcing, spalling, and other adverse factors, such as electrode fractures and electrode losses within the melt.
For electric arc furnaces, the consumption caused by lateral oxidation may range up to 40% of the total electrode consumption, while the tip consumption may range up to 60%.
Various ways have been suggested to lower the graphite electrode consumption. Tip consumption is due to the normal use within the furnace; and fractures and other similar losses, such as from arcing, can be minimized through adequate control of the electrode height. Numerous efforts have been particularly made to solve the additional significant problem caused by lateral oxidation losses. One method of reducing graphite electrode consumption has been the utilization of non-consumable metallic electrodes which have a cooled non-consumable upper portion, at whose bottom portion is affixed a graphite tip by means of a nipple. This type of electrode has the disadvantage of requiring special insulation to avoid arcing with other conductive parts, e.g., with steel scrap, as well as numerous problems in its use caused by the insulation and some difficulties in transferring electrical energy.
Another suggested way to decrease the graphite electrode consumption requires the application of external protective coatings of materials resistant to oxidation. These coatings cause an increase in the contact resistance between the electrode and the power clamp, which causes a decrease of the energy transferred to the melt and consequently an increase on the energy required to melt down the charge. Compound electrodes of the type disclosed on U.S. Pat. Nos. 4,490,824 and 4,498,185 are known.
U.S. Pat. No. 4,490,824 discloses a compound electrode provided with two concentric passages arranged vertically along the axis of the electrode. This electrode has the disadvantage of lacking sufficient cooling of its lateral surfaces, leaving unsolved the lateral consumption problem. Moreover, the cooled portion is non-consumable and requires special nipples.
U.S. Pat. No. 4,498,185 discloses a compound electrode within which a plurality of copper tubes, radially arranged, extend longitudinally along the electrode and are serially interconnected to allow the coolant fluid to pass therethrough. This type of electrode is non-consumable and its type of interconnection prevents homogeneous cooling.